1. Technical Field
The present disclosure relates to a grinding wheel for use in ferrous roll grinding applications and a method to regrind rolls to desired geometrical quality. The disclosure also relates to grinding wheels comprising cubic boron nitride as the primary abrasive in a bond system.
2. Description of the Related Art
Rolling is a forming process used to produce strips, plates, or sheets of varying thickness in industries such as the steel, aluminum, copper and paper industries. Rolls are made to varying shapes (profiles) with specific geometric tolerances and surface integrity specifications to meet the needs of the rolling application. Rolls are typically made out of iron, steel, cemented carbide, granite, or composites thereof. In rolling operations, the rolls undergo considerable wear and changes in surface quality and thus require periodic re-shaping by machining or grinding, i.e., “roll grinding”, to bring the roll back to the required geometric tolerances while leaving the surface free of feed lines, chatter marks and surface irregularities such as scratch marks and/or thermal degradation of the roll surface. The rolls are ground with a grinding wheel traversing the roll surface back and forth on a dedicated roll grinding machine (off-line) or as installed in a strip rolling mill with a roll grinding apparatus (on-line) attached to the roll stand in a mill.
The challenge with both of these methods is to restore the roll to its correct profile geometry with minimum stock removal and without visible feed marks, visible chatter marks or surface irregularities. Feed lines or feed marks are imprints of the wheel leading edge on the roll surface corresponding to the distance the wheel advances per revolution of the roll. Chatter marks correspond to wheel-work contact lines that occur periodically on the circumference of the roll either due to wheel run out error or due to vibrations that arise from multiple sources in the grinding system such as grinding wheel imbalance, spindle bearings, machine structure, machine feed axes, motor drives, hydraulic and electrical impulses. Both feed marks and chatter marks are undesirable in the roll, as they affect the durability of the roll in service and produce an undesirable surface quality in the finished product. Surface irregularities in the roll are associated with either a scratch mark and/or thermal degradation of the working surface of the roll following grinding. Scratch marks are caused by either loose abrasive particles released from the wheel or grinding swarf material scratching the roll surface in a random manner. A visual inspection of the roll is normally used depending on the application to accept or reject the roll for scratch marks. Thermal degradation of the roll surface is caused by excessive heat in the grinding process resulting in a change in the microstructure of the roll material at or near the ground surface and/or sometimes resulting in cracks in the roll. Eddy current and ultrasonic inspection methods are employed to detect thermal degradation in the rolls following grinding.
Typically for an off-line roll grinding method, a grinding machine is equipped such that the grinding wheel rotational axis is parallel to the work roll rotational axis and the rotating wheel in contact with the rotating roll surface is traversed along the axis of the roll back and forth to produce the desired geometry. Roll grinding machines are commercially available from a number of vendors that supply equipment to the roll grinding industry including Pomini (Milan, Italy), Waldrich Siegen (Germany), Herkules (Germany), and others. The grinding wheel shape used in off-line roll grinding is typically a Type 1 wheel, wherein the outer diameter face of the wheel performs grinding.
It is common practice in the roll grinding industry to grind iron and steel roll materials with grinding wheels comprising conventional abrasives such as aluminum oxide, silicon carbide, or mixtures thereof, along with fillers and secondary abrasives in an organic bonded resin wheel system, e.g., a shellac type resin or a phenolic resin matrix. It is also known in the industry to use diamond as the primary abrasive in a grinding wheel made with a phenolic resin bonded matrix to grind roll materials made of cemented carbide, granite or non-ferrous roll materials. Inorganic bonded or vitrified or ceramic bonded abrasive wheels have not been successful in roll grinding applications compared to organic resin bonded wheels, because the former has a low impact resistance and low chatter resistance compared to the latter. The organic resin bonded wheels are known to work better in roll grinding applications because of their low E-modulus (1 GPa-12 GPa) compared to inorganic vitrified bond wheels, which have a higher E-modulus (18 GPa-200 GPa). Another problem associated with the vitrified bonded conventional wheel system is that its brittle nature causes the wheel edge to break down during the grinding process, resulting in scratch marks and surface irregularities in the work roll.
U.S. Pat. App. Pub. No. 20030194954A1 discloses roll grinding wheels consisting essentially of conventional abrasives such as aluminum oxide abrasive or silicon carbide abrasive and mixture thereof, agglomerated with selected binder and filler materials in a phenolic resin bond system to give improved grinding wheel life over a shellac resin bond system. In the examples, a cumulative grinding ratio G of 2.093 after grinding 19 rolls is demonstrated, representing an improvement of 2-3 times the G observed for shellac resin bonded wheels. The grinding ratio G represents the ratio of volume of roll material removed to the volume of wheel worn. The higher the value of G, the longer the wheel life. However, even with these improved grinding wheels the rate of grinding wheel wear is still quite large in grinding steel rolls, that continuous radial wheel wear compensation (WWC) is employed during the grind cycle to meet geometrical taper tolerances (TT) in the roll. In the art, taper tolerance TT corresponds to the allowable size variation in the roll from one end of the roll to the other end. WWC is done by continually moving the grinding wheel feed axis into the roll surface as a function of the axial traverse of the wheel. The requirement of WWC in roll grinding dictates the need for sophisticated machine controls as well as added complexity to the grinding cycle.
There is a second disadvantage with the grinding wheels employing conventional abrasives of the prior art. The wheels undergo rapid wheel wear during the roll grinding process, requiring multiple corrective grinding passes to generate both a roll profile and taper within the desired tolerance, which is typically less than 0.025 mm. These additional grinding passes result in the removal of expensive roll material, leading to a reduction in the useful work roll life. Typically in the prior art, the ratio TT/WWC ranges from 0.5 to 5 (where TT and WWC are expressed in consistent units) to meet roll specifications with conventional abrasives. A higher ratio of TT to WWC is particularly desirable to maximize the useful roll life and grinding wheel life, and thus improve the efficiency of the roll grinding process.
The third disadvantage of corrective grinding passes is increased cycle time, thus reducing the productivity of the process. Loss of productive time also occurs due to frequent wheel changes that result from accelerated wear of the organic resin bonded wheels. Yet a fourth disadvantage faced with conventional abrasive wheels is that the useful wheel diameter typically decreases from 36-24 inches (914-610 mm) over the life of the wheel, the compensation for which can result in a large cantilever action of the grinding spindle head. The continuous increase in cantilever action results in continually changing stiffness of the grinding system, causing inconsistencies in the roll grinding process.
A number of other prior art references, i.e., European patent documents EP03444610 and EPO573035 and U.S. Pat. Nos. 5,569,060 and 6,220,949, disclose an on-line roll grinding method, Japanese patent document JP06226606A discloses an off-line roll grinding apparatus and operation, wherein a planar disk face wheel (a cup face wheel) Type-6A2 is used to grind the roll. The grinding wheel axis in this type of grinding system is perpendicular to work roll axis, such that the axial side face (working face) of the wheel is pressed with a constant force in frictional sliding contact with the outer circumferential roll surface. In this design, the wheel spindle axis is tilted slightly so that contact with the work roll surface occurs on the leading face of the wheel. The grinding wheel in this method is either passively driven with the aid of torque of the work roll, or positively driven by a grinding spindle motor.
In another prior art reference, European patent document EP0344610 discloses a cup face wheel used in on-line roll grinding having two abrasive annular ring members integrally bonded, wherein the wheels comprise aluminum oxide, silicon carbide, CBN or diamond abrasives in two different bonding systems such as organic or inorganic bond system for each abrasive member respectively. The vitrified bonded abrasive layer (having higher E-modulus 19.7-69 GPa) is the inner ring member; and the outer ring member is made with an organic resin bonded system (lower E-modulus 1-9.8 GPa) to avoid chipping and cracking of the wheel. As the rates of grinding wheel wear are not the same for the two members of different bonding systems, profile errors, chatter, and scratch marks may frequently be experienced in grinding the roll.
U.S. Pat. Nos. 5,569,060 and 6,220,949 disclose a cup face phenolic resin bonded CBN wheel with different flexible wheel body design to absorb the vibrations induced in the rolling mill stands while grinding the work roll. With a flexible wheel body design herein, the contact force between the wheel face and roll surface is typically controlled at a constant magnitude (between 30-50 kgf/mm width of the grinding wheel face) during the grinding process to achieve uniform contact along the working wheel face.
This type of flexible wheel design is also applied in the off-line grinding method disclosed in Japanese patent publication JP06226606A. Grinding with a constant wheel flexure or a constant wheel load with a cup face grinding wheel means that the material removal rate depends on the sharpness of the wheel and the type of roll material that is being ground. Since the wear on the work roll in the mill operation is not always uniform, it can be very challenging when the work roll wear is large (in excess of 0.010 mm) as non-uniform contact between the cup wheel face and the roll surface develops. This results in uneven wheel wear, affecting the cutting ability or the sharpness of the wheel along its working face, causing uneven stock removal in the work roll along its axial length and resulting in profile errors and chatter in the process.
A stable grinding process with a cup face CBN grinding wheel is then possible by frequently grinding the rolls and correcting the surface irregularities before a large wear amount develops on the roll. With this approach, it is conceivable that the ratio TT/WWC can be increased beyond 10 compared to the conventional abrasive Type 1 wheel that is used in the off-line grinding method. A limiting factor of the cup face wheel design, however, is that it can present considerable challenge and difficulty in keeping the ratio TT/WWC greater than 10 when grinding rolls of various shapes such as a convex crown, concave crown or a continuous numerical profile along the axis of the roll.
The off-line and on-line roll grinding methods offer two different approaches to resurface the work rolls and back up rolls with their different kinematic arrangements and grinding process strategies. The grinding article used in the off-line method is used to grind a single work roll material specification, or more often multiple work roll material specifications such as iron, high speed steel-HSS, high chromium alloy steel, etc., during the useful life of the wheel. On the other hand, the on-line wheel grinds only a single work roll material specification that is used in that stand over the life of the wheel. Therefore, grinding wheel article specifications and wheel manufacturing methods used for making a cup face planar disk wheel (Type 6A2) design cannot be translated to making a Type 1 grinding wheel as their application methods are significantly different.
As mentioned earlier, grinding without chatter marks and feed marks are extremely important in grinding mill rolls. Japanese Patent No. JP11077532 discloses a device to grind rolls without chatter. In this device, vibration sensors mounted on the grinding spindle head and the roll stand continuously monitor the vibration level during the grinding process and adjust the grinding wheel and roll rotational speeds such that it does not exceed a threshold chatter vibration level. This method, however, requires that the speed ratio between the revolution speed of the grinding wheel and the revolution speed of the roll be kept constant, which adds complexity in grinding a good quality roll.
There is a need for an improved and simplified roll grinding method to grind the work rolls of various profile shapes and ferrous material specifications with a single wheel specification such that the ratio TT/WWC is greater than 10. Maximizing TT/WWC ensures significant cost savings in expensive roll materials. There is also a need for a grinding wheel having improved grinding wheel life to improve roll quality, thereby reducing the total consumable cost in the roll shop and in the strip mill.
The disclosure contained herein describes attempts to address one or more of the problems described above.